This invention relates to a method of wafer/substrate bonding, particularly but not exclusively, to a method of anodic bonding.
Wafer/substrate bonding has increasingly become a key technology for materials integration in various areas of microelectromechanical systems (MEMS), microoptoelectromechanical systems (MOEMS), microelectronics, optoelectronics and substrate fabrication. It is also widely used for vacuum packaging, hermetic sealing and encapsulation.
In order to bond the wafers/substrates together, various technologies have already been developed, for example, silicon fusion bonding, anodic bonding and intermediate layer bonding, including eutectic, glass frit, solder and adhesive bonding.
Anodic bonding is widely used for bonding between a glass wafer/substrate and a metal or semiconductor wafer/substrate, such as a silicon wafer. Using the silicon wafer as an example, in a conventional anodic bonding process, the wafers are heated, typically to a temperature between 300° C. and 600° C., which increases the mobility of the positive ions in the glass. Next, a typical voltage of 400 volts to 1200 volts is applied to the glass and the silicon wafer to be bonded, such that the voltage of the silicon wafer is positive with respect to the glass wafer. The positive ions in the glass are attracted to the high negative voltage thus creating a space charge at the glass-silicon interface, which produces a strong electrostatic attraction between the silicon and glass wafers, fixing them firmly in place.
The elevated temperature also causes oxygen from the glass wafer to be transported to the glass-silicon interface where the oxygen combines with the silicon to form silicon oxide, creating an irreversible chemical bond at the glass-silicon interface, when the voltage is subsequently removed.
Therefore, a higher temperature generally yields a better bonding strength. Preferably, a bonding temperature above 400° C. is recommended, which has been demonstrated to achieve a successful bond. However, a high bonding temperature may result in other problems such as a residual stress after the anodic bonding due to the thermal expansion mismatch between the two wafers, and damage of the metal circuits on the silicon wafer after the bonding. In addition, a high temperature may prevent the use of certain metals in the bonding stack as well.
On the other hand, if the bonding temperature is reduced, bonding quality will be affected resulting in a weak or poor bond between the two wafers. In addition, a poor bond is susceptible to the formation of bubbles or cavities in the glass-silicon interface, which are difficult to minimize or eliminate.
There have been proposed methods of bonding between a glass wafer/susbtrate and a silicon wafer/substrate at relatively low bonding temperatures. For example, patent number U.S. Pat. No. 5,695,590 discloses a method of bonding two glass wafers using an intermediate layer of alkaline ion barrier between the two wafers at a temperature between 250° C. and 500° C. However, a problem with this proposed method is that bonding quality is relatively poor at the lower limit of the disclosed temperature range. Patent document U.S. Pat. No. 5,820,648 relates to an anodic process between a silicon and a glass substrate at a temperature of approximatley 200° C. by irradiating a light beam to relax the network structure of glass so as to promote the diffusion of modifier ions in the glass. A disadvantage of this proposed method is that it is difficult to irradiate the entire bonding area of the substrates, especially for wafer level bonding, since the electrode to supply the required voltage may shield the light beam. Moreover, the apparatus to perform this proposed method is expensive to setup.
It is an object of the invention to provide a bonding method which alleviates at least one of the disadvantages of the discussed prior art.